Design and prototyping of braodband metasurface scalar phase masks for high-contrast imaging

NASA’s future Habitable Worlds Observatory (HWO) will enable the direct detection and characterization of Earth-like planets around Sun-like stars using high-contrast imaging. One of the most promising approaches to achieve this goal is to use a coronagraph. A good candidate for implementation in HWO is the vortex coronagraph, which is featured in both original mission concepts HabEx and LUVOIR. However, at the high contrast levels required for HWO (1e-10), the polarization dependency and chromatic leakage of the vector vortex coronagraph becomes a limiting factor, making it challenging to perform wavefront control in both polarizations simultaneously. Filtering one polarization or splitting the two polarizations provide possible solutions, but at the expense of reducing the throughput by a factor of 2, or doubling the number of optics in the instrument. An alternative approach is to use a scalar vortex phase mask, which imprints the same phase ramp onto both polarizations. However, the broadband performance of the scalar vortex has to be improved to meet the 1e-10 contrast goal of HWO. Metasurfaces present a promising technology for achromatizing scalar vortex coronagraphs, because they allow for more design freedom in a single layer of constant thickness compared to scalar vortex phase masks based on variable thickness of a dielectric substrate. Here, we present our progress in developing metasurface scalar vortex phase masks. We present updated broadband designs of scalar metasurface phase masks of different topographies, and simulate their performance with appropriate simulation tools. We also discuss first manufacturing attempts at such masks, and outline the next steps needed to push their performance towards the levels required for HWO.